1. Field of the Invention
The invention relates generally to image signal processing and in particular to a method and apparatus for compensating for fixed pattern noise in image sensor image signals.
2. Background of the Prior Art
When pixel values, representative of pixel voltages are read from an image sensor pixel array, slight inconsistencies are observable between the voltages of the various pixels even when the array is exposed to a scene of uniform radiance. The inconsistency in the pixel values read from the various pixels is commonly referred to as “noise” either of a fixed pattern noise type or a random noise type.
Fixed pattern noise refers to inconsistencies in the responsiveness of an image sensor output that results from sensor characteristics or properties affecting pixels of the sensor in the same way each time data is captured from those pixels. Random or stochastic noise refers to inconsistencies which vary each time data is captured from the pixels.
While random or stochastic noise cannot be compensated, it is useful to adapt an imaging system in video signal generating or decoding applications to automatically compensate pixel values that deviate from patterns that might be expected based on the array's radiance pattern.
Various problems have been noted in prior art imaging systems configured for fixed pattern noise compensation, both in set up routine methods for configuring an imaging system for fixed pattern noise compensation, and in methods for compensating pixel values read from a pixel array.
In one known setup routine for configuring an imaging system to compensate for fixed pattern noise, a pixel array is exposed to uniform “dark frame” by shielding the pixel array from light. The pixel values read from the array during exposure to the dark frame are then used to determine pixel offset terms. This method can be employed to estimate an additive component of fixed pattern noise, but cannot be used to determine a multiplicative component of fixed pattern noise. Furthermore, the setup method involving exposure to a dark frame is susceptible to measurement clipping. If a change in incident radiance does not result in a change in pixel value, then clipping has occurred.
In a known analog hardware method for compensating voltages read from a pixel array, hardware components are provided and selectively activated to compensate pixel voltages shifted out of a pixel array. A major problem with this scheme is that values read from the array can be corrected only to a small number of discrete voltages, thereby limiting the precision of the correction. Also, this scheme requires additional hardware.
Failure to compensate adequately for fixed pattern noise has significant consequences, for example, when an imaging system is employed in indicia decoding applications. In such applications, it is common to define an area of interest in a captured image before decoding algorithms commence. Fixed pattern noise has been observed to cause a symbology decoding apparatus to misplace edge positions, and to result in erroneous identification of edge positions. The likelihood of an erroneous edge position identification increases when a decoding apparatus is employed to capture symbols having higher densities.